1. Field of the Invention
The present invention relates to a distributed amplifier having an input network that simulates an input signal transmission line, and having an output network that simulates an output signal transmission line, and also having multiple unit cells with amplifying characteristics that are connected in parallel to one another between the input network and the output network, that amplify a signal propagating through the input network and feed it into the output network with a predetermined phase relationship to a signal propagating through the output network.
2. Description of the Background Art
Amplifier structures, which are known as “distributed amplifiers” or “traveling wave amplifiers,” are known, for example from the publication, “A Novel HBT Distributed Amplifier Design Topology Based on Attenuation Compensation Techniques,” IEEE Transactions on Microwave Theory and Techniques, Vol. 42, No. 12, December 1994. The input networks and output networks are also known as “artificial transmission lines” or “synthetic transmission lines.” Their task is to absorb parasitic elements of the unit cells. In this context, absorption is understood to mean that the phase-shifting action of parasitic elements, for example that of input and output capacitances of the unit cells, is altered by phase-shifting components matched thereto in the aforementioned networks such that the input signals amplified by the active cells are fed into the output network with identical phase to a signal propagating in the output network, thus amplifying the signal.
Individual transistors or circuits with multiple transistors are known as unit cells. Thus, for example, the aforementioned publication shows unit cells identified there as “gain cell topologies” with a “conventional bipolar transistor,” an “emitter follower/common emitter,” or an “emitter follower/cascode with negative feedback.” According to this publication, an improvement in the gain-bandwidth product of 200% as compared to conventional common-emitter distributed amplifier configurations is achieved with the structures introduced there.
The goal for distributed amplifiers in general is the largest possible product of large bandwidth and gain. In this regard, this product is limited by the effects of parasitic capacitances, especially by the input capacitances ci of the unit cells, by a signal attenuation occurring in the input network, and by a signal attenuation occurring in the output network.
Thus, the cutoff frequency fci resulting from the input capacitances obeys the relationship:
            f      ci        =          1              π        ⁢                                            C                              i                ⁢                                                                  ⁢                n                                      ⁢                          L                              i                ⁢                                                                  ⁢                n                                                          ,where Lin represents the inductive component of the input network, which is to say of the artificial input transmission line.
The signal attenuation occurring in the input network and that occurring in the output network limit, in particular, the number of unit cells that can usefully be connected in parallel for an increase in the output power and a maximum gain bandwidth product.
A similar relationship applies to the cutoff frequency of the output network. Since the capacitances acting at the input of the unit cells are generally larger than the capacitances acting at the output, the cutoff frequency and thus the bandwidth of the distributed amplifier is generally dominated by effects on the input side.
In an ideal case of loss-less input and output networks, the gain follows the relationship
      G    =                            n          2                ⁢                  g          m          2                ⁢                  Z          0          2                    4        ,where Z0 indicates the characteristic impedance of the input network and/or output network, gm indicates the transconductance of a unit cell, and n indicates the number of unit cells. In the ideal case, the gain G can be increased by increasing the number of unit cells. In reality, however, the number of unit cells that can usefully be employed is limited by the aforementioned losses in the input and output networks.